A satellite provides its own power for the duration of its mission, which can extend to ten years or more. The most common source of power for Earth-orbiting satellites is a combination of solar cells with a battery backup. Solar cells need to be large enough to provide the power that the satellite requires. For example, the solar array of the complex Hubble Space Telescope is about 3,120 sq ft in area and generates about 5,500 watts of electricity, while the solar array of a smaller Global Positioning System satellite is about 50 sq ft in area and generates about 700 watts of electricity. Solar cells are often mounted on winglike panels that unfold from the body of the satellite after it reaches its final orbit. Batteries provide power before the solar panels are deployed and when sunlight does not reach the solar panels.
A satellite’s orientation is the direction each of its sides face. The satellite keeps the solar panels pointed toward the Sun. In addition, the satellite’s antennas and sensors point toward Earth or toward the object the satellite is observing. For example, communications and weather satellites have antennas and cameras pointed earthward, while space telescopes are pointed toward the astronomical objects that scientists wish to study. Methods of maintaining orientation include small rocket engines, known as attitude thrusters; large spinning wheels that turn the satellite; and magnets that interact with Earth’s magnetic field to correctly orient the satellite. Attitude thrusters can make large changes to orientation quickly, but they are not the best solution when the stability of the turn is critical. Attitude thrusters also require fuel, so the lifetime of the satellite depends on a limited supply of fuel for the thrusters. A spinning wheel on a satellite acts as a gyroscope. The rotational motion of the wheel makes the satellite stay in one orientation, and changing the rotational motion will cause the satellite to turn. Spinning wheels and magnets are slower than thrusters but are excellent for attitude stability and require only electric power.
As it orbits Earth, a satellite encounters intense heat and intense cold as it alternately faces or is hidden from the Sun. The electronic equipment on the satellite also creates heat that can cause damage. On Earth, convection, conduction, or radiation of heat can transfer heat. With no air flowing over the satellite to transfer heat by convection and no body to which the satellite can conduct heat, the satellite must radiate heat to control temperature. Often satellites use radiators in the form of louvered panels, including panels that open and close to adjust the amount of radiating surface area. To prevent the direct rays of the Sun from causing hot spots, the satellite may spin or rotate to distribute the Sun’s heat more evenly.
Cosmic Radiation and Micrometeoroid Protection
Satellites have to endure the effects of radiation and of continuous, damaging micrometeoroid hits, especially during long-term missions. Earth’s atmosphere blocks most cosmic radiation from affecting microprocessors in computers on the ground. A satellite, however, needs shielding for its computers. Radiation from space also causes some materials to become brittle, so parts of satellites break more easily after long exposure to the electromagnetic radiation of space. Solar panels gradually produce less and less power because of damage from radiation effects and from the impact of micrometeoroids.
Many military satellites are similar to commercial ones, but they send encrypted data that only a special receiver can decipher. Military surveillance satellites take pictures just as other earth-imaging satellites do, but cameras on military satellites usually have a higher resolution.
The U.S. military operates a variety of satellite systems. The Defence Satellite Communications System (DSCS) consists of five spacecraft in geostationary orbit that transmit voice, data, and television signals between military sites. The Defence Support Program (DSP) uses satellites that are intended to give early warning of missile launches. DSP was used during the Persian Gulf War (1991) to warn of Iraqi Scud missile launches.
Some military satellites provide data that is available to the public. For instance, the satellites of the Defence Meteorological Satellite Program (DMSP) collect and disseminate global weather information. The military also maintains the Global Positioning System (GPS), which provides navigation information that anyone with a GPS receiver can use.
The US’s Strategic Defence Initiative (SDI), announced by President Reagan in March 1983, revitalised and harmonised research into active defence means - including ground-based, airborne and spaced based sensor and weapon systems.
The aim to move away from the single option of retaliation and, at the very least. To buy time to make key decision-makers. Despite the money poured into SDI, however, no operational strategic defence systems were produced or fielded.
The planned US National Missile defence (NMD) system is a radar-controlled, anti-ballistic missile (ABM) system, designed to shoot down long-range, nuclear or possibly chemical or biological armed missiles. Such systems are banned under the 1972 ABM Treaty between the US and the Soviet Union (Russia).
Space Defence Weapons
Space-Based Directed Energy Weapons
Space-based directed energy weapons have a number of attractive features that make them a leading candidate for boost-phase engagement weapons. Beams travelling at the speed of light (300,000 km/sec) from lasers, or at near light speed from particle
beam weapons, can rapidly deposit lethal fluencies of energy on ballistic missile targets at ranges of thousands of kilometres. There are at least two concepts for producing such short wavelength laser beams.
Excimer lasers utilize Excited Dimers, such as Xenon and Chlorine. They are electrically stimulated to emit coherent light at wavelengths in the near ultraviolet region of the spectrum. This provides a higher lethality against hardened targets than is possible using infrared lasers. However, because these lasers have very high power requirements) they have not been a leading candidate for space-based systems.
Free electron lasers (FELs) operate by interacting high energy electron beams with magnetic wiggler fields to convert the electron beam kinetic energy into optical radiation. Compared to Excimer lasers, FELs offer simpler power conditioning requirements and a relatively mature technology base derived from work on electron beam accelerators. Because of their wavelength selectability and relatively high electrical efficiency, FEL devices are promising candidates for space-based systems.
Depending on the brightness of the laser device, from several dozen up to a hundred FEL battle stations, each weighing several hundred tons would be required for an operational constellation. A number of configurations have been suggested for such weapon platforms
"... four separate but coherently-phased FEL devices with the output beams combined onto a common single-output aperture... to minimize the distance (170 m) between the forward and aft resonators positioned on either side of the respective accelerator /wiggler assemblies. If the total output power were to be generated by a single FEL device, that 170 m distance would grow to an estimated 600 m which would exceed the bounds of reason for a total required platform length."
Neutral particle beam weapons could destroy boosters, decoys and re-entry vehicles through structural damage. Weapons of lower power could be used to negate nuclear warheads by damaging nuclear and electronic components.
Alternative Space-Based Weapon Concepts
The hallmark of the SDI since 1983 has been an initial layer of space-based interceptors that home in on the hot exhaust plumes of hostile missiles during the first few minutes of their flight. This boost-phase layer is intended to destroy missiles before they can deploy multiple warheads and decoy warheads that would stress
the performance of subsequent layers of the defence.
Originally, plans for this layer of the system called for Space-Based Interceptor (SBI) rockets, each weighing about 100
kilograms, with between five and ten interceptor rockets carried on a satellite that would also carry target-tracking sensors. The 1987 plan called for approximately 3,000 interceptors to be carried on approximately 300 Carrier Vehicle satellites, while the 1988 plan called for about 1,500 interceptors deployed on about 150 Carrier Vehicle satellites.
A major change in these plans came in early 1989 with adoption of
the "Brilliant Pebbles" (BP) concept (the name implying improved capabilities compared with the SBI "smart rocks"). Each Brilliant Pebble would orbit separately, making a less attractive target for Soviet attack. This dispersal, as well as advanced construction techniques, would also permit each Brilliant Pebble to weigh about
40 to 50 kilograms, less than half that of the traditional SBI. Each Brilliant Pebble would have its own missile tracking sensors, eliminating the need for the BSTS satellite sensor. And computers on-board each Brilliant Pebble would direct each Pebble to its target, reducing reliance on expensive communications systems for
ground control. The initial plan for Brilliant Pebbles called for 4,614 to be procured at a cost of between $1.1 million and $1.4 million each.
Space-based chemical lasers have also been considered for the boost-phase mission. Unlike the previously discussed directed energy devices, which require large amounts of electrical power
for laser beam generation, chemical laser beam generators produce laser light though the combustion of reactants, such as deuterium and fluorine. A deployed space based laser derived from the Triad technology would weigh on the order of 100,000 kilograms, and have a mirror 15 meters in diameter. The laser would have a power
output of about 25 Megawatts, and carry sufficient fuel for about 100 seconds of operations. The brightness of the system would be on the order of 1.0 X 10^23 watts /steradian.
Ground-Based Directed Energy Weapon Concepts
Although space-based directed energy weapons have received extensive study, by the late 1980s increasing attention was being given to ground-based systems. High performance short wavelength systems include a concept where a beam of about 10 Megawatts would be generated on the ground and propagated to one or more mirrors in space, and then focused on the target. For a space-based system, a total of between 10 and 40 mirrors, each with a diameter of about 10 meters, would be required in orbits of about 1000 kilometres altitude. Alternatively, a very large number of mirrors could operate in unison, much like a phased array radar, to increase the energy deposited on the target. This would permit ground basing of the mirrors to enhance survivability. The mirrors would be launched into space on warning of attack. Target destruction would either be by thermal or impulse kill.
A ground-based laser weapon system would ultimately consist of a number of ground sites where high energy laser devices and the appropriate acquisition, tracking, pointing, and advanced beam control subsystems to provide the compensation necessary to transmit the laser beam through the atmosphere to space through "bounces" from space relay and mission mirrors to the target. Pointing and tracking accuracies of about 20 nanoradians would be required.
Such system architectures achieve the access to boost-phase of space based systems by basing mission mirrors in space, but reduce the cost, maintenance and survivability problems associated with basing beam generators in space.
The Strategic Defence Initiative includes work on ground-based, air-based and space-based sensors for the surveillance, acquisition, tracking and kill assessment of ballistic missiles in all phases of their flight: boost phase; post-boost phase; mid-course; and terminal. These sensors would be required to perform their duties with high confidence in the face of an attack consisting of thousands of missiles, tens of thousands of warheads, hundreds of thousands of decoys, and billions of bits of chaff and aerosols.
Surveillance requires the continuous coverage of likely missile launch locations and regions in space through which missiles and their warheads would pass in order to reach their targets.
Acquisition requires that targets be identified as such in the presence of natural background noise and hostile interference, and that threatening objects such as warheads are discriminated from non-threatening decoys.
Tracking these targets requires that their precise location and trajectory be determined and frequently updated so that interceptor forces can be properly assigned. Target tracking also assists in the further discrimination of targets from decoys.
Kill assessment is required to determine whether a target has in fact been negated, or whether further action is called for. The different responses of warheads and decoys to some types of directed energy weapons such as X-Ray lasers allows the kill assessment function for further contribute to the decoy discrimination effort.
The surveillance and acquisition processes are intended to be conducted independently by each sensor. This will increase the resistance of the system to direct attack. Tracking and kill assessment functions will be conducted on a consultative basis; that is, the observations from different sensors will be combined and compared. This consultation will increase the confidence in the assessment, enhance decoy discrimination, and provide continuous target coverage throughout the engagement regime. SDI sensors are divided into three classes: passive, active and interactive. The primary difference among these classes is their potential for discrimination during mid-course.
Passive sensors (such as infrared detectors) are used for surveillance, and tracking, as well as discrimination, by detecting thermal signature differences between targets and decoy. For example, during mid-course, light balloon decoys will cool off
much more rapidly that heavy warheads, and this temperature difference can be detected. However, offence countermeasures, such as placing small heaters in the empty balloons or surrounding the threat complex with aerosols that will mask the position and signature of re-entry vehicles, have a significant potential to defeat this form of discrimination. Power requirements for such sensors range from a few kilowatts up to 100 kilowatts, depending on the size and complexity of the infrared detector systems.
Active sensors (such as radar or ladar) are used for tracking, and
discrimination of real targets from decoys by emitting a signal that is modified when it reaches the target or decoy, with targets producing different changes in the signal than are produced by decoys. For example, some types of decoys will produce a much
weaker reflection of a radar beam than is produced by a warhead. Power requirements for such large space-based sensor systems can range from 1 MWe for laser radars to 5 MWe for space based microwave radars.
Interactive sensors (such as a neutral particle beam generator of an X-Ray Laser) are used for discrimination by generating emissions that interact with a target or decoy, with targets changing in a fashion that is observably different from the changes in the decoy. For example, a neutral particle beam will penetrate through several centimetres of aluminium before it interacts with aluminium atoms to produce secondary radiation. The NPB will thus produce observable secondary radiation when it strikes a warhead, which is much thicker than this depth, but it will pass through the very thin
skin of a balloon decoy without interacting and producing observable secondary radiation.